It has been found that certain asymmetric composite laminates exhibit bistability, where the composite laminate exhibits multiple stable static equilibrium states. If the bistable composite is actuated, it will snap to its secondary equilibrium state and then remain there without further actuation. This study investigates how the amount of symmetry in a combined symmetric asymmetric rectangular laminate under an imposed clamped edge boundary condition affects the bistability and the curvature of the laminate. Laminates with varying amounts of asymmetry were fabricated and then measured using a profilometer to capture the curvatures of the equilibrium shapes. The results showed that up to 20% symmetry can be introduced in the laminate without a substantial loss in snap through curvature, and that up to 83% symmetry can be introduced in the laminate before bistability is lost. Finite element simulations were conducted in Abaqus and showed good correlation with the experimental results.
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Defining relationships between geometry and behavior of bistable composite laminates
Bistability is exhibited by an object when it can be resting in two stable equilibrium states. Certain composite laminates exhibit bistability by having two stable curvatures of opposite sign with the two axes of curvature perpendicular to each other. These laminates can be actuated from one state to the other. The actuation from the original post-cure shape to the second shape is called as ‘snap-through’ and the reverse actuation is called as ‘snap-back’. This phenomenon can be used in applications for morphing structures, energy harvesting, and other applications where there is a conflicting requirement of a structure that is load-carrying, light, and shape-adaptable. MW Hyer first reported this phenomenon in his paper in 1981. He found that thin unsymmetric laminates do not behave according to the predictions of the Classical Lamination Theory (CLT). The CLT is a linear theory and predicts the post-cure shape of thin unsymmetric laminates to be a saddle. MW Hyer developed a non-linear method called the “Extended Classical Lamination Theory” which accurately predicted the laminate to have two cylindrical shapes. Since then, a number of researchers have tried to identify the key parameters affecting the behavior of such laminates. Geometric parameters such as stacking sequence, fibre orientation, cure cycle, boundary conditions, and force of actuation, have all been studied. The objective of this research is to define a relation between the length, width and thickness of square and rectangular laminates required to achieve bistability. Using these relations, a 36 in × 36 in bistable laminate is fabricated with a thickness of 30 CFRP layers. Also, it is proved that a laminate does not lose bistability with an increase in aspect ratio, as long as both sides of the rectangular laminate are above a certain ‘critical length’. A bistable laminate with dimensions of 2 in × 50 in is fabricated. Further, for laminates that are bistable, it is necessary to be able to predict the curvature and force required for actuation. Therefore, a method is developed which allows us to predict the curvature of both stable shapes, as well as the force of actuation of laminates for which the thickness and dimensions are known. Finite Element Analysis is used to carry out the numerical calculations, which are validated by fabricating laminates. The curvature of these laminates is measured using a profilometer and the force of actuation is recorded using a universal test set-up.
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- Award ID(s):
- 1760943
- NSF-PAR ID:
- 10290956
- Date Published:
- Journal Name:
- Journal of Composite Materials
- ISSN:
- 0021-9983
- Page Range / eLocation ID:
- 002199832110058
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Bistable composite laminates have large-scale applications in morphing and energy-harvesting structures, but their fatigue performance remains largely unexplored. This study investigates the stiffness and damage progression and evaluates bistable performance to develop protocols for long-term applications. We analyze the effects of displacement-controlled fully reversible high cycle fatigue-loading on stiffness, damage, curvature, and snap-through load in the out-of-plane loading direction at eight different combinations of parameters with frequency from 1 to 10 Hz, two boundary conditions, and temperature from 22°C to 150°C up to 3 to 10 million cycles. Stiffness and damage evolution analysis demonstrate the first two stages in out-of-plane fatigue loading. The study proposes a damage definition in terms of load adapting with two fatigue damage models: (1) Shiri Model and (2) Wu Model, while both models exhibit reasonable accuracy in predicting damage for the first two stages despite deviating at the final cycle due to assuming this cycle as the final failure cycle. Of the two models, the Shiri model provided a smaller range of model parameter values, 0.22 and 0.43, for parameters p and q, respectively, which reflects adjustability to different test conditions by maintaining a moderate range. Specimens encountered no final failure by fiber breakage and did not lose bistability for any combination. Curvature and snap-through load measurements have not substantially changed due to fatigue loading. These findings confirm application protocols with a broad range of parameters for which the laminates can operate without significant fatigue damage and maintain their bistable performance for an infinite lifetime.
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1177/00219983211005824
